Lateral loads on timber frames, pole barns etc. 3

[HeavyCivil]

I sometimes am required to design or detail open structures (usually just accessories to structures like canopies over entry ways, small generator enclosures etc.) which end up as pole-barn style with knee braces.

I am usually under pressure to use no braces from architects. I am trying to figure out how to design "wood moment frames". My understanding of how to determine allowable moments and loads of traditional joinery is pretty poor (and also beyond the scope of what I need to do-I've never been asked by a contractor is they could just use a mortise and tenon to join kiln dried lumber instead of the fasteners I spec'd). However it would be practical to know what I could get away with without using knee braces. I would really like to determine what forces and moments timber frames + fasteners can take. I often assume pinned connections even when there is some or much fixity (my impression is many people do this, and as long as you account for the fact that the connection likely does carry some moment I think its okay, but conservative). But I'd like to, in certain cases, use this fixity to my benefit and rely on it.

So for example - say you have a carrier beam supported by columns connected with 16d's and a column cap. How would you determine the moment capacity there? Without braces or shear walls you'd have to determine the same thing for the column base at the ftg. Any thoughts?

 

[HeavyCivil]

Edit: I guess if your frame (or braces or shear walls) resist lateral loads you do not need to worry about rotation at base.

 

[BAretired]

Moment connections in timber are a pain in the neck, but here are some references which may be of help:

http://www.sigi.ca/engineering/courseware/Moment Resisting Timber Connections.pdf

http://www.eng-tips.com/viewthread.cfm?qid=133826&page=9

http://staff.fit.ac.cy/eng.ma/BATECH08/Connections.pdf

BA

 

[HeavyCivil]

BA - great links, thank you.

As a follow up question (this may be kind of elementary) a rigid diaphragm really does not serve much of a purpose in a pole-barn style structure, does it? I cant see a case where, in using a carrier beam and tied rafters or trusses, there is any benefit from sheathing instead of purlins.

So in lieu of knee braces or shear walls, It seams that moment resisting wood connections are the only options, but according the the referenced threads, they are not a very good one.

 

[OHIOMatt]

Have you looked at cantilevered columns? This is a fairly common approach to post frame buildings and will work well on small structures.

 

[Splitrings]

VTEIT,
We use ASABE reference on shallow post design. I tried to attach the link. The post is modeled as a cantilever, being fixed at a point approximately the embedment depth/3 or calculated as do/2. As long as your load at the top of the post isn't too great this would be a cleaner looking pole type building. Obviously with K factor of 2 instead of 1, your post size may increase.

I have only modeled a wood connection as fixed if it was cast in a sizable concrete section. I have my doubts that you can create a wood connection with fasteners that approaches a true fixed connection. Although, I haven't looked at BAretired's links yet.

 

[HeavyCivil]

OHIO - yes, and I think its a good idea. If the post-base can be fixed then there is no need for wood moment connections. The moment connection is between the wood column,steel base pl and concrete footing. Still - I'm fuzzy on designing this and I think it gets into the same problems as with the wood-wood connection - bolt pattern and spacing could exceed space on an 8x8 quick, I would guess.

Splitrings- I agree that casting In is a better option (and easier to analyze) than a bolted connection. My concern there would be how to provide a durable barrier between a PT column and the concrete. Maybe this what Ohio was getting at.

I guess its a matter of engineering judgment on how thick and wide a section the post is cast into justifies the fixed connection.

 

[Splitrings]

VTEIT,
We have been specifying that our posts be treated to the UC4B Category of the AWPA standard. This category applies to utility poles, building poles and permanent wood foundations in contact with the ground. There is also category UC4C that is for even more extreme environments. I don't have the complete standard or any structures that have been in long enough to evaulate.

 

[msquared48]

I would not use a rigid diaphragm analysis for pole type structures. Stick with the tributary area method. These usually have a plywood diaphragm roof, if at all, and any metal roof is usually non-structural.

If the structure or the load gets too large, a plywood diaphragm roof and plywood shear walls may not be an option, but necessary. I have done knee braces, but you have to coordiinate the connections with any truss manufacturer as connections are required at the top and bottom chords. True that Architects do not like them, but some clients like to save money too. In fact, show me one who doesn't.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask